1. Field of the Invention
High temperature insulating materials which are suitable, for example, as reusable reentry heat shield for orbiting vehicles.
2. Description of the Prior Art
The development of a reusable space vehicle has created a need for a reusable surface insulating material. The characteristics required by such a material include mechanical strength and high strain to failure, as well as resistance to devitrification or crystallization at high temperature which would render the insulation inferior for reuse.
Conventionally, the high temperature insulations of the art have been formed by bonding ceramic fibers with inorganic binders. Aluminosilicate and silica fibers have been taught as appropriate for use in such composites (U.S. Pat. Nos. 3,077,413 and 3,752,683). The fibers, however, are always interspersed in a matrix of binder, usually an inorganic oxide such as colloidal silica. The use of a binder matrix limits the potential of the individual fibers in the resultant insulation to a fraction of their possible strength or strain to failure. The present invention is unique in that it dispenses with the binder matrix which the prior art considers necessary; the present materials are composed of fibers only.
The production of an acceptable insulation composed of fibers alone is even more of a surprise to the art in light of the fibers which are used to produce this new invention, one of which being the highly sensitive, high purity silica fibers. Since small amounts of impurities, it has been taught, can cause undesirable divitrification in such fibers, it could reasonably have been inferred from these teachings that the other fiber ingredient of this invention, aluminoborosilicate fibers, would act as such an impurity and cause devitrification of the silica fibers. This has now been shown not to be the case.